T5 
870 


UC-NRLF 


U.  S.  Department  of  Agriculture,  Forest  Service 

FOREST  PRODUCTS  LABORATORY 

In  cooperation  with  the  University  of  Wisconsin 

MADISON,  WISCONSIN 

• 


NOTES  ON  THE  MANUFACTURE  OF  PLYWOOD 


Revised 
September,  1922 


Aerie .  -  Forestry .  M  ain  Library 


NOTES  ON  THE  MANUFACTURE  OP  PLYWOOD 


By  manufacture  of  plywood  is  meant  the  gluing 

t 

together  of  plies  of  wood,  usually  an  odd  number,  so 
laid  that  the  grain  of  alternate  layers  is  approximately 
at  right  angles*    In  three-ply  panels  the  outside  plies 
are  referred  to  as  faces   and   the   center  ply  as  a  core. 
In  a  five-ply  construction  the  outside  layers  are  faces, 
the  next  two  plies  in  order  cross~bands  and  the  center  ply 
core.  The  details  of  gluing,  pressing,  and  drying  plywood 
are  not   standardized,  but  these  notes  on  manufacture  are 
"based  upon  observations  of  factory  practice  and  upon  exten- 
sive experiments  at  the  Forest  Products  Laboratory. 

Gluing  Panels  in  General 

The  core   or  crossbands  of  the  panels  are  coated 
with  glue   on  a  glue  spreader  consisting  of  two   rollers, 
the  lower  one  revolving  in  a  bath  of  glue  and  the  upper  one 

sometimes  being  coated  from  another  bath  of  glue  retained 

> 

between  the  upper   roller  and  a  trough  with  its  lower  edge 
almost  touching  the  surface  of  the  roller.   Scrapers  or  the 
pressure  of  the  rolls  regulate  the   thickness   of  the  glue 
layer  as  desired. 


L26  519081 

54-3  -i 


The  panels  are  built  up  and  placed  in  the  press 
as  soon  as  possible,   The  actual  time  consumed  "between 
spreading  glue  on  the  first  panel  and  the  application  of 
pressure  to  the  stack  of  panels  varies  greatly  in  different 
factories  from  a  few  minutes  to  a  half  hour  or  longer.   The 
panels  are  usually  grouped  with  three  to  five  in  a  set  with 
caul  "boards  placed  between  sets  and  at  the  top  and  bottom  of 
the  entire  lot  before  pressure  is  applied.   The  panels  are 
usually  left  under  pressure  for  7  to  24  hours  and  then  taken 
to  the  drying  room.   Shorter  pressure  periods  can  be  used  in 
some  cases  when  necessary. 

Two  different  methods  are  used  rather  extensively 
in  applying  the  pressure  and.  maintaining  it  on  the  panels „ 
The  one  perhaps  most  commonly  used  consists  in  applying  the 
pressure  by  a  hydraulic  press  and  then  using  retaining  clamps 

to  keep  the  panels  under  pressure*   The  hydraulic  press  is 

» 

usually  equipped  with  a  pressure  gauge  which  shows  the  amount 
of  pressure  applied.   The  panels  are  left  in  the  hydraulic 
press  just  long  enough  to  apply  the  proper  load  and  fix  the 
retaining  clamps  in  place.   The  bundle  of  panels  is  then  re- 
moved on  a  truck  to  an  out  of  the  way  place  in  the  factory 
where  they  are  usually  left  until  the  next  day. 


L26 

543  .2- 


By  the  other  method  the  panels  are  placed  in 
presses  and  left  until  the  glue  is  set,   !£hese  presses  are 
usually  of  the  hand  screw  type  with  no  means  for  measuring 
accurately  the  amount  of  pressure  applied, 

V/i th  glues  of  the  blood  albumin,  type,  demanding 
the  use  of  a  hot  press,  the   glue   is  applied  as  in  the 
case  of  cold  glue  and  the  panels  placed  one  or  more  at  a 
time   in   the  hot  press.    Pressure  up  to  200  pounds  per 
square  inch  and  temperatures  of   212   degrees   Fahrenheit 
or  more  are  usually  applied.   The  panels  are  kept  in  the 
press  until  the  glue  is  set,  varying  from  two  to  several 
minutes,  depending  upon  the  thickness  of  the  plies  and  the 
panels  and  the  ty^e  of  cauls  used. 

Calculation  of  Pressure  in  a  Hydraulic  Veneer  Press 

For  plywood  manufacture  determination   of   the 
correct  amount  of  pressure  to  use  is  important.   Results 
of  experiments  at  the  Forest  Products  Laboratory  indicate 
that  an  excess  or  a  lack  may^  produce  a  weak  glue  joint, 
The   exact  amount   of  pressure  to  apply  per  square  inch  of 
panel  surface  varies  with  a  number  of  conditions.   However, 
pressures  of  75  to  1JO  pounds  per  square  inch  of  panel  sur- 
face are  ordinarily  within  the  range  of  which  good  results 
may  be  obtained  with  most  glues  and  under  average  factory 
conditions.   When  animal  glue  chills  in  a  cold  room  before 


L26 
54-3 


pressure  is  applied,  very  he^vy  pressures  are  then  neces- 
sary to  secure  strong  joints.  In  this  case  pressures  of 
400  pounds  per  square  inch  or  more  are  required, 

The  determination  of  the  amount  of  pressure  apr 
per  square  inch  of  panel  on  a  Iiyo.rau3.io  press  ecu.lpped 
with  a  pressure   gauge   is  simply  a  matte:.-  of  calculation,. 
'Vith  any  hydraulic  press,  thi3  depends  upori  three  factors: 
The  area  of  the  panel,  the  area  of  the  p. I:-: ton  or  ram  of  the 
press,  and  the  pressure  gauge  reading,'5*   The  area  of  the 
piston  in  square  inches  multiplied  "by  the  pressure   gauge 
reading  in  pounds  is  approximately  equal  to  the  total  preb«. 
sure   exerted  "by   the  plates.    The  tot'al  pressure   ex-., 
erted  divided  by  the  area  of  the  panel  in  square  inches 
gives  the  pressure  secured  on  the  pare],  in  pounds  per  square 
inch. 

The  pressure  gauge  reading  musit.  therefore,  vary 
with  the  size  of  the  panels  if  the  same  amount  of  pressure 
per  square  inch  is  to  "be  secured,   I'cr  example,  if  the  same 
gauge  pressure  is  applied  on  panels  .10  by  ']6  inches  as  o:.i 
others  36  by  40  inches  the  pressure  -;:  ]  1  >>e  "approximately 
four  times  as  great  per  square  inch  :ln  ~,ho  first  case  as  in 
the  second. 


It  is  assumed  of  course  that  the  gauge  is  in  good  repair 
and  indicating  the  pressure  correctly.   Pressure  gauges 
should  "be  checked  occasionally  to  see  that  they  are  correct 

\ 

L26 

$43  -4- 


Prom  the   considerations   above  the  following 
formulas  for  the   calculation  of  pressures  are  derived! 

P  =  P1'*  A'      (1)* 
A 

or 

pr  .  P  x  A       (2)* 

A 

Where  P  =  gauge  pressure  in  pounds  per  square  inch. 

P'z  pressure  on  panels  in  pounds  per  square  in. 
A  s  area  of  piston  or  ram  in  square  inches. 
A'r  area  of  panel  'in  square  inches. 

To  illustrate  the  use  of  the  formulas  let  the  fol- 
lowing case  be  assumed:   On  a  hydraulic  press  with  10-inch 
piston  what  pressure   gauge  reading  is  necessary  to  secure 
75  pounds  per  square  inch  on  panels  24  by  48  inches?  Use 
formula  (l) 


* 

To  determine  accurately  the  exact  prossure  secured  the 
weight  of  the  platen  to  which  the  pressure  is  applied  must  "be 
taken  into  consideration.   For  practical  purposes,  however, 
it  may  "be  omitted  from  the  calculation-,  as  inaccuracies  in 
the  gauge  reading,  etc.,  may  account  for  much  larger  errors. 
When  the 'weight  of  the  platen  is  taken  into  consideration 
the  formulas  "become: 

•P  ~  P*  x  A*-  plus  or  minus  W   (3) 

A  A 

or 

P f  -  P  x  A  minus  or  plus  ¥.  (4) 
A'  A' 

Where  W  r  weight  of  the  lower  platen  plus  weight  of  panels 
or  upper  platen  alone,  as  the  case  may  be*   The  sign  of  the 
last  member  of  equation  (3)  is  plus  when  the  pressure  is 
applied  by  the  lower  platen  and  minus  when  applied  by  the 
upper.   In  equation  (4)  the  reverse  is  true, 

L26 

543  -?- 


-      r 
\    -'• 


P  =  P'  x  A' 
A 

Here  P  «:  gauge  reading  required 

?'  ^  75 

A?  E  24  jr  43  or  1152  (area  of  panel) 

A  ~  3,  141  6  x  jk  or  78»54,  the  area  of  the 


... 
piston*-.. 

Thus   P      :;  Z.L-2--JL4-5.2   or  H30    The    required   gauge 

78,5--  reading. 

A  table,  showing  gauge  readings  to  "be  used  for 
all  sized  panels  manufactured  and  for  the  different  pres- 
sures used,  can  "be  computed  and  placed  r.ear  the  press  ?;here 
the  operator  may  see  at  a  glance  the  amount  of  pressure 
required  on  the  gauge  reading  for  each  run  of  panels. 

Drying 

Panels  take  up  a  good  deal  of  moisture  in  gluing 
and  after  coming  from  the  press  are  usually  placed  on  stick- 
ers and  run  into  a  kiln  or  left  at  room  conditions  for  final 
drying*   Dryingunder  room  conditions  is  blow  arid,  "because  of 
the  space  required,  is  expensive,   The  use  of  panel  kilns 
has  "become  more  common  of  late  years.   The  necessity  for 
quick  drying  of  panels  in  connection  v/i  t.h  aircraft  work  has 
aided  considerably  in  this  development,, 


The  area  of  a  circle  is  equal  to. 3.1416  times  the  square 
of  the  radius. 

L26 

543  -6- 


Results  of  experiments  in  kiln  drying  aircraft 
panels  have  indicated  that  the  essential  requirements  of 
minimum  injury  to  the  material,  a  sufficiently  rapid  dry- 
ing rate,   and  convenience  and   economy  of  operation  can 
"best  Toe  met  "by  maintaining  a  constant  temperature   of 
about  120  degrees  Fahrenheit  and  a  constant  maximum  rela- 
tive humidity  which  will  permit  the  stock  to  dry  down  to 
the  final  degree  of  dryness  desired,  but  which  will   not 
all'ow  an  appreciable  amount  of  further  drying  "below   the 
required  moisture  content.   The  use  of  constant  tempera- 
ture and  humidity  conditions  which  will  dry  the  panels  to 
a  definite  moisture  content  makes  the  drying  simple,  safe, 
and  easy.   For  panels,  of  three  and  five-ply  veneer  or  of 
veneer  faces  and  cros standing  and  a  thick  core  and  glued  at 
a  low  moisture  content,  drying  at  120  degrees   Fahrenheit 
and  the  necessary  maximum  humidity  may  "be  accomplished  in 
from  a  few  hours  to  over  night,,    Temperatures  above  120 
degrees  Fahrenheit  have   the  advantage  of  decreasing  the 
drying  time  but  are  more  liable  to   lowsr  the   quality  of 
the  panel  "by  inducing  checking,  warping,  and  open  joints, 
Panels  dried  from  a  high  to  an  excessively  low  moisture 
content  are  very  liable  to  warp  and  should  be  dried  rela- 
tively slowly. 


L26 

543  -7- 


Gluing;  "by  a  Dry  Glue  Proc esjs. 

In  connection  witli  aircraft  work  a  process  of 
laying  veneer  with  dry  blodd  glue  was  worked  out  at  the 
Tores t  Products  Laboratory,  "   It  consists  of  making  an 
.adhesive  by  coating  a  tissue  paper  with  "blood  albumin  glue, 
allowing  it  to  dry,  and  using  the  coated  paper  as  the  glue  . 

layer  for  plywood*    Ths  sheets  of  glue  are  alternated 

• 
with  sheets  of  veneer  until  the  required  number  of  plies 

is  obtained  and  the  whole  is  pressed  in  a  hot  press.   The 
use  of  the  dry  glue  enables  the  construction  of  plywood 
with  the  addition  of  little  or  no  moisture.    Veneers  as 
thin  as  1/lpO-inch  can  be  glued  successfully  into  thin 
sheets  of  plywood  orpanels.  .  ' 

The  process  can  be  used  with  thicker  stock  but 
appears  to  be  adapted  especially  to  fancy,  cross -grained, 
and  very  thin  veneer.   It  should  also  prove  successful  with 
a  number  of  other  adhesives  than  blood  glue.   The  method 
eliminates  a  number  of  troubles  result ir.g  from  the  use  of 
wet  glues,  such  as  checking,  warping,  open  joints,   and 
overlaps,  and  makes  the  handling  of  thin  iiaterial  much 
easier. 


* 

At  least  two  patents  on  similar  process  have  been  dis- 
covered since  that  time,  one  a  British  patent  ITo.  17,32? 
issued  in  1902  and  the  other  U.S.  patent  No.  1,299,74?  issued 
in  1919.   The  value  of  these  patents'  is  not  definitely  known, 
but  it  is  suggested  that  anyone  interested  in  producing  this 
material  should  familiarize  himself  with  them  in  order  to 
avois  infringement. 
L26 
543  -8. 


Factors  Affecting  the  War-rin^  of  Plywood 

S  ymni e  t  r  i  c  al  Construct.!  on , 

On  account  of  the  great  difference  in  shrinkage 
of  wood  in  the  direction  parallel  to  the  grain  and  perpen- 
dicular to  itr  a  change  in  moisture  content  of  plywood  will 
inevitably  either  introduce  or  .'-..relieve  internal  s.tresses. 
Take,  for  example,  a  three-ply  construction  and  subject  it 
to  low  humidity  conditions  so  that  the  moisture  content  of 
the  plywood  is  lowered,   Because  the  grain  of  the   core 
is  at   right  angles  to  the  grain  of  the  faces,  the  core 
will  t:end  to  shrink  a  grea,t  deal  more  than  the  faces,  in 
the  direction  of  the  grain  of  the  faces0   -  This  shrinkage 
subjects  the  faces  to  compression  stresses  and   the   core 
to  tensile  stresses.    If   the   faces  are  of  exactly  the 
same  thickness,  of  like  density  and  otherwise  "balanced  the 
stresses  are  symmetrically  distributed  and  no  cupping  should 
ensue, 

Now  let  itbe  assumed  that  one  face  of  a  three- 
ply  panel  has  "been  glued  with  the  grain  in  the  .same,  di- 
rection as  the  core,  and  that  the  mois'tare  content  of  the 
panel  is  reduced.   It  is  obvious  that  the  internal  stresses 
are  now  no  longer  symmetrically  distributed,  inasmuch  as 
the  compressive  stresses  in  one   face  have  been  removed. 
This  face  now  shrinks  a  great  deal  more  than  the  other  face 
in  the  direction  of  the  grain  of  the  latter.    The   result 
is  that  cupping  takes  place. 

126 

54-3  -9- 


The  necessity  for  exercising  care  in  sanding  the 

faces  of  a  panel  is  obvious,  inasmuch  as  different  thick- 

4 

nesses  on  the  faces  would,  introduce  unequal  forces   with 
changing  moisture  content  and  produce  distortion. 

In  order  to  obtain  symmetry  it  is  also  necessary 
that  "both  faces  of  symmetrical  plies  "be  of  the  same  species, 
or  species  of  approximately  the  same  properties > 

To  summarize!   A  plywood  panel  to  retain  its 
form  with  changes  of  moisture  must  "be  symmetrically  con- 
structed.  Symmetry  is   obtained  "by  using  an  odd  number 
of  plies.    The  plies  should  be  so  arranged  that  for  any 
ply  of  a  particular  thickness  there  is  a  parallel  ply  of 
the  same  thickness   and  of  the  same  species  o:c  properties 
on  the  opposite  side  of  the  core  and  equally  removed  from 
the  core. 

Direction  of  the  Grain  of  the  Plies 

b       In  the  discussion  of   symmetry  of  construction 
it  was  understood  that  the  successive  plies  were  always 
glued  with  the  grain  either  parallel  or  exactly  at  right 
angles  to  the  core.   In  careless  construction  this  may  not 
always  be  the  case.    Tests  have  shown  that  deviations  as 
small  as  five  degrees  from  the  standard  90-degree  con- 
struction may  introduce  considerable  twisting. 


L26 

54-3  -10- 


Jri  building  up   a   three-ply  vsieer  panel    the 
•» 
core- should,  "be   glued  with   the   grain   at    90   degrees   with 

the  faces,    or  as   close    to    this   as   feasible, 

Mai H t ur e   C  c  n t r  ol 

The  previous  discussion  brought  out  the  fact 
tha£  a  change  in  moisture  content  of  a  panel  may  intro- 
duce cupping  and  tv/i  sting  in  the  panel  if  it  is  not  care- 
fully constructed.   Hence  it  is  highly  desirable  that  all 
plies  "be  at  about  the  same  moisture  content  before  glu- 
ing, and  that  the  moisture  content  of  the  panel  when  it 
leaves  the  drying  room  should  be  about  the  same  as  it 
will  average  when  in  use.   The  limits   of  from  10  to  15 
per  cent  moisture  in  the  finished  panel  will  usually  give 
satisfactory  results  when  the  panel  is  in  service  in  the 
open  airv   For  use  in  heated  buildings,  at  least  a  part 
of  the  year,  as  in  furniture  a  somewhat  lower  moisture  con. 
tent  of  sevenrto  eight  per  cent  will  ordinarily  give  best 
results. 

Veneer  for  furniture  panels  should  generally  be 
low  in  moisture,  so  that  when  removed  from  the  press  the 
moisture  content,  increased  by  the  moisture  from  the  glue, 
will  be  as  near  as  possible  to  that  required  for  factory 
use.   For  fancy,  cross-grained  veneer  the  gluing  at  low 


L26 

54-3  -11- 


moisture  content  is  of  particular  importance,  since  dry- 

4t 

ing  of  the  panel  frequently  results  in  checking  of  the 
face,   Non-water-res  is tnnt  glues  also  require  the  use  of 
dry  veneer,  lout  with,  casein  and  "blood  glues  it  is  possible 
to  use  wet  veneer. 

Good  strength  and  water  resistance  can  be   ob- 
tained with  these  water-resistant  glues  with  veneer  at  a 
moisture  content  as  high  as  50  per  cent,,   For  maximum  wet 
strength  of  plywood  the  veneer  should  be  glued  at  a  moisture 
content  which  would  bring  the  panels  to  about  3°  Per  cent 
or  more  when  removed  from  the  press.   The  high  moisture 
content  of  the  panels  made  in  this  way  necessitates  con- 
siderable care  in  drying  if  a  good  quality  panel  is  to  be 
secured,  and  it  is1  improbable  that  much  use  can  be  made  of 
this  method  of  gluing  in  furniture  manufacture.   It  offers 
possibilities,  however,  for  cutting  cos'cs  in  the  manufacture 
of  articles  where  glue  strength,  water  resistance,  or  speed 
of  manufacture  is  more  important  than  appearance, 

Relation  .of  Densj.tv-_  of  Veneer  to  Warping 

Numerous  tests  have  shown  that  the  warping  of  ply- 
wood panels  when  subjected  to  varying  moisture  contents  is 
least  for  the  panels  made  of  low  density  veneer,  such  as  . 
basswood,  poplar,  and  cedar,  and  that,  in  general,  warping 
increases  with  increasing  density,, 


L26 

54-3  -12- 


Effect  on  Warding  of  Increasing  the  Ratio  of  the  Core 
to  the  Total  Plywood  Tliicknes.3. 

A  high  proportion  of  core  to  total  plywood 
thickness  helps  to  maintain  a  flat  unwarped  surface.   In 
general,  the  core  should  comprise  5/10  to  7/13  of  the 
total  thickness  of  the  panel  where  flatness  is  an  impor- 
tant consideration.   Of  three-ply  panels  having  cores  of 
the  same  weight  the  panels  having  cores  of  low  density, 
such  as  poplar  and  "basswood,  will,  in  general,  show  less 
warping  than  the  panels  having  high  density  cores,  such - 
as  maple  and  "birch. 


L26 

54-3  -13- 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


1 


AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  $1.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


FEB     9    1935 


FEB  23  1935 


a 


4 


1  MatTSOMW 


----- r-  --- 


-MOV- 


DEC  1 9  1950 


OEC5   »G7 


1  2 


LOAN  DEPT 


LD  21-100m-8,'34 


